MongoDB Interview Questions and Answers: 60 Questions from Easy to Hard

db.orders.createIndex({ status: 1, createdAt: -1, amount: 1 })

// Equality, Sort, Range: this index fits
db.orders.find({ status: "paid", amount: { $gt: 1000 } })
         .sort({ createdAt: -1 })

// no status in the filter: this index does not help
db.orders.find({ createdAt: { $gte: ISODate("2025-04-01") } })

db.orders.find({ city: "Pune", status: "paid" })
         .explain("executionStats")

// executionStats (trimmed):
//   nReturned:          120
//   totalKeysExamined:  18000
//   totalDocsExamined:  18000
//   stages: IXSCAN { city: 1 } -> FETCH (filter: status)

// jobs
{ _id: 1, title: "Backend", skills: ["node", "mongodb"],
  cities: ["Pune", "Noida"] }

db.jobs.createIndex({ skills: 1 })            // multikey
db.jobs.createIndex({ skills: 1, title: 1 })  // fine
db.jobs.createIndex({ skills: 1, cities: 1 }) // fails: two arrays

db.employees.createIndex({ empCode: 1, name: 1 })

db.employees.find({ empCode: "PUN-0042" },
                  { name: 1, _id: 0 })
// covered: totalDocsExamined 0

db.employees.find({ empCode: "PUN-0042" },
                  { name: 1 })
// _id is returned too, so each match is fetched

// customers: { _id: 1, name: "Priya" }
//            { _id: 2, name: "Rahul" }
//            { _id: 3, name: "Sneha" }
// orders:    { _id: 101, custId: 1, amount: 900 }
//            { _id: 102, custId: 1, amount: 450 }
//            { _id: 103, custId: 3, amount: 1200 }
db.customers.aggregate([
    { $lookup: { from: "orders", localField: "_id",
                 foreignField: "custId", as: "orders" } },
    { $project: { name: 1,
                  orderCount: { $size: "$orders" },
                  spent: { $sum: "$orders.amount" } } },
    { $sort: { _id: 1 } }
])

// posts
{ _id: 1, tags: ["mongodb", "nosql"] }
{ _id: 2, tags: ["mongodb"] }
{ _id: 3, tags: [] }
{ _id: 4 }

db.posts.aggregate([ { $unwind: "$tags" } ])          // A
db.posts.aggregate([ { $unwind: { path: "$tags",
    preserveNullAndEmptyArrays: true } } ])           // B
db.posts.aggregate([
    { $unwind: "$tags" },
    { $group: { _id: "$tags", n: { $sum: 1 } } },
    { $sort: { n: -1, _id: 1 } }
])                                                    // C

// results
{ _id: 1, name: "Asha", scores: [72, 91] }
{ _id: 2, name: "Ravi", scores: [83, 60] }
{ _id: 3, name: "Neha", scores: [55, 65] }

db.results.find({ scores: { $gt: 80, $lt: 85 } })      // A
db.results.find({
    scores: { $elemMatch: { $gt: 80, $lt: 85 } }
})                                                     // B

// carts: { _id: 7, coupons: ["NEW10"], items: [
//     { sku: "TEA", qty: 1 },
//     { sku: "RICE", qty: 2 },
//     { sku: "DAL", qty: 0 } ] }
db.carts.updateOne({ _id: 7 },
    { $addToSet: { coupons: { $each: ["NEW10", "FEST5"] } } })
db.carts.updateOne({ _id: 7, "items.sku": "RICE" },
    { $inc: { "items.$.qty": 1 } })
db.carts.updateOne({ _id: 7 },
    { $pull: { items: { qty: 0 } } })
db.carts.updateOne({ _id: 7 },
    { $set: { "items.$[i].gift": true } },
    { arrayFilters: [ { "i.qty": { $gte: 2 } } ] })

db.counters.findOneAndUpdate(
    { _id: "invoice-2025-26" },
    { $inc: { seq: 1 } },
    { upsert: true, returnNewDocument: true }
)
// first call returns { _id: "invoice-2025-26", seq: 1 }

// page 500 at 20 per page: walks past 9,980 entries
db.jobs.find({ city: "Pune" })
       .sort({ _id: -1 }).skip(9980).limit(20)

// keyset: pass the last _id of the previous page
db.jobs.find({ city: "Pune", _id: { $lt: lastId } })
       .sort({ _id: -1 }).limit(20)

db.jobs.createIndex({ city: 1, _id: -1 })

db.jobs.createIndex(
    { title: "text", description: "text" },
    { weights: { title: 5, description: 1 } }
)
db.jobs.find(
    { $text: { $search: "java developer -intern" } },
    { title: 1, score: { $meta: "textScore" } }
).sort({ score: { $meta: "textScore" } })

// seat holds expire 10 minutes after createdAt
db.seat_holds.createIndex({ createdAt: 1 },
                          { expireAfterSeconds: 600 })
db.seat_holds.insertOne({ show: "S-21", seat: "F12",
                          createdAt: new Date() })

// or expire at a time stored in each document
db.passes.createIndex({ expiresAt: 1 },
                      { expireAfterSeconds: 0 })

db.users.createIndex({ email: 1 }, { unique: true })
db.users.insertOne({ name: "Asha" })   // ok, no email
db.users.insertOne({ name: "Ravi" })   // E11000 duplicate key

db.users.dropIndex("email_1")
db.users.createIndex({ email: 1 }, {
    unique: true,
    partialFilterExpression: { email: { $type: "string" } }
})

db.createCollection("employees", {
    validator: { $jsonSchema: {
        bsonType: "object",
        required: ["empCode", "name", "joinDate"],
        properties: {
            empCode:  { bsonType: "string",
                        pattern: "^EMP[0-9]+$" },
            name:     { bsonType: "string", minLength: 2 },
            joinDate: { bsonType: "date" },
            deptId:   { bsonType: "objectId" }
        }
    } },
    validationLevel: "strict",
    validationAction: "error"
})

// Node.js route GET /api/jobs/:id
// broken: a string never equals an ObjectId
await jobs.findOne({ _id: req.params.id });       // null

// fixed: validate, then convert
const { ObjectId } = require("mongodb");
const id = req.params.id;
if (!/^[0-9a-f]{24}$/i.test(id)) {
    return res.status(400).json({ error: "bad id" });
}
const job = await jobs.findOne({ _id: new ObjectId(id) });

// budgets
{ _id: 1, dept: "Sales", budget: 500000, spent: 620000 }
{ _id: 2, dept: "IT", budget: 900000, spent: 450000 }
{ _id: 3, dept: "HR", budget: 200000, spent: 200000 }

db.budgets.find({ $expr: { $gt: ["$spent", "$budget"] } })   // A
db.budgets.find({ $expr: { $gte: ["$spent", "$budget"] } })  // B

// Express login handler (vulnerable)
app.post("/login", async (req, res) => {
    const user = await users.findOne({
        username: req.body.username,
        password: req.body.password
    });
    if (user) return res.json({ ok: true });
    res.status(401).end();
});
// the attacker posts this JSON body:
// { "username": "admin", "password": { "$ne": "" } }